Quick-Change Tool Post (QCTP)Adapted a
Phase2+ wedge-type QCTP,
Model 250-111.
Wedge-type tool posts are purportedly
more accurate & repeatable
than the piston type. Quick-Change Tool Posts allow fast tool changes,
exact tool bit height& position/angle adjustments.A long,
22mm Craftsman box wrench is used
to
loosen & tighten thehardened tool post
nut for rotational positioning. Full-sized lathes have a large T-slot to mount the tool post. The Jet BD 920N is too small for
a T-slot but this
mod parallels that design.
The new design uses a machined M14-1.5 x
120mm long bolt (same size as the
Phase2+ OEM post) with a counter-bored cross slide.
To keep the bolt head as large as possible, it was turned
down just enough to
remove the hex flats.
Also replaced
all the gib
M4-0.7 x10mm
set screws with higher
quality US parts. The lock
nuts are stainless steel. Replaced the three compound bearing plate
Philips-head
mounting screws with stronger,
hex cap-head bolts. These bolt heads had to be ground down enough to
allow
sufficient clearance when the plate is clamped down to the cross slide.
Phase2+ wedge-type Quick-Change Tool Post Set
manual.

Old modification design (top), M14-1.5 x 120mm bolt (middle) & new design (bottom).
My first QCTP modification (some years ago, before I had a
larger mill
& a boring head), was accomplished
by turning down the Phase2+ OEM 14mm post to fit into the original
cross slide hole, but it was a weak design.

Used
a (centered) 1-in. end mill to cut a relief in the dovetail to keep the
bolt head as large as possible.
This clearance cut to the dove tail does not seem to have affected the
compound's mechanical
performance.
The counter bore is about 0.83" & the clearance hole is a very close fit to the
14mm bolt.
The counter-bore depth is the same as that used by the original bolt post.
To allow complete seating into the hole, the bolt-head & counter-bore edges were
properly chamfered.
The machined bolt head is an interference fit so it had to be hammered in using a
brass rod.

This was the first test part fabricated & it was flush to the
surface. I made a second part with a longer bolt.
Also, the head was machined a little thicker (not flush) for more strength & it used the available
2mm slide clearance area between the bottom of the slide & the top of the
dove-tail base.
Note the small amount of brass that was left on t he bolt head surface from the
installation.

Used a
rotary
table to mill the four corners of the compound's raised, square tool post platform.Rounding the
corners allows the QCTP dovetail
tool holder to be adjusted lower than the surface of the platform,
thereby enabling
adjustment of larger tools (e.g.,
scissors knurling, cutoff tool,
1/2"
bits) at thecenterline of the work
piece.

Tool post bottom is shown
with a tool holder in place. The holder dovetail corners have to clear the base.

Used the
Phase2+ OEM hardened, flanged nut.
Maximum tool bit size is 1/2". The
increased rigidity is outstanding.
When combined with a negative-rake carbide insert
tool bit holder, steel-cutting performance has dramatically improved.I have now
increased the size of the
compound pivoting base making it about 50% larger than the OEM in all
dimensions.

This 5/8" (15.875mm)
wide, negative 5 deg rake tool bit used to put high stress on this machine but now
that
it has
been upgraded with a variable
speed DC motor & timing belt/pulley system, it easily makes no-chatter
cuts.

Negative-rake tool-bit holders in QCTP height-adjustable dovetail mounts.
These use TNMG-style, 1/8" thick, zero relief
angle inserts which have six usable tips, each.
The middle holder is an MTENN
type having an insert with a 1/32" radius tip.

Different
radius insert tips can be used e.g., 1/64" or 1/32". The holders have 1/2" hardened shanks.TNMG-style inserts are the most economical to use due to six tips, apiece.
Their popularity & high industry demand drives their cost down.

Click on linked thumbnailsTool Bit Holders
QCTP
tools left to right,
top row: an
MT2 arbor holder
with a drill chuck, 3/4" &
5/8" (with a
split insert) boring bar
holder, a
conventional knurling tool (also holds a bit),
& two, scissors-type knurlers. Left to right,
bottom row:
small boring bar, a cutoff blade holder
& tool bit holders. Some of the tool-bit holders have a V-groove to
hold
a round shank like those found on a
3/8" boring
bar. The cutoff tool holder was ground along the inner
top corner to relieve the
wider top of some cutting blades. The blade's side sits
flush against the holder.

Note this
Phase2+ tool
holder has a V-groove to hold round shanks.

This
3/8" round to
1/2" square, slit, steel adapter eliminates
set screw marring on
the shaft.The hole was reamed for a close, precision fit to the ground shank.

Left piece parted with a little cutting oil whereas
the right part was cut-off using lots of
oil. Of course, cutting oil
greatly extends toolbit life.

Click on linked thumbnails

T-bar Cutoff ToolAdapted the
Taig T-bar
cutoff tool to work on the 9x20. The blade is positioned at the back & upside down.
With a front cutoff tool, forces push the blade downward below
centerline & into the part possibly causing
chatter. With a back cutoff tool, forces pull the blade upward
above centerline & away from the part possibly
reducing chatter. For max rigidity during cutoffs, keep the
blade as short & close to the chuck as possible.

Mounted rear view - An aluminum guide indexes the tool post.
The OEM T-bar design had only two,
plate-clamping 10-32 bolts, so I added two more for additional
strength.
Since the blade clamp plate is symmetrical, I just rotated it &
used a transfer punch to locate the extra bolts.

The front base M6-1 bolt (right) is forward of the
post bolt for maximum leverage against the upward pull.
The rear base bolt (left) uses a hardened washer. Two, M6-1
steel T-nuts are
used in the cross-slide slot.

A center significantly increases setup
rigidity.

The side plate on the left is held on by four, counter-bored 8-32 hex-head screws.

The side plate functions as a bracket-in-tension
against upward pulling forces during cutting.
A good example of a bracket-in-tension is that of a playing
card; easy to crush but hard to pull apart.
The top edge of the side plate contacts the bottom edge of the
blade clamp to lend additional support.
Opened-up the two clearance holes for the main 6mm bolts to 6.7mm to
allow small, rotational adjustments.
The rigidity of this setup is
very good; no chatter, 50-400 RPM in aluminum.

Cutoff performance is enhanced especially
now that the lathe can turn low RPM using
the powerfulvariable-speed
DC motor & timing belt pulley setup. This cutoff operation is being
performed
at 60
RPM but easily works up to 400 RPM.
Absolutely no chatter & the cuts are clean.
The variable speed DC motor driven spindle has plenty of torque
throughout its 50-1000 RPM range.
It is my belief that, in part, cutoff-induced chatter could have
been
due to macroscopic fluctuations of the
spring-loaded idler pulley when under heavy spindle loads. The
removal of the OEM idler belt/pulley
system & subsequent replacement with a strong timing belt has
eliminated cutoff-induced chatter.

Tool Bit Height
GagesAluminum height gage used to quickly set tool
bit height using a QCTP.
Right side for standard
settings & left side
for upside down,
backside bits.
Facing cuts were made on a
collet-held piece until no center nub remained.
That height
operationally defines the tool bit height for the gage. A
Dorian
Tool, negative rakeTNMG
carbide insert tool bit holder
MTGNR is
shown. The
height-gage
bottom rests
on the cross slide.

Another quick tool bit height setting method is to gently pinch a
small rule between the tip & the work piece, then adjust the height until it is vertical. If the rule's
top tilts toward you, then it is low & vice versa.